This invention relates generally to radio frequency antennas and more particularly to small, compact, radio frequency antennas.
As is known in the art, one type of radio frequency antenna is a so-called "spiral antenna". Such antenna is generally characterized by a pair of dipole conductors which spiral inwardly from an outer portion of the antenna to an inner portion of the antenna. The inner ends of each of the conductors are disposed adjacent one another and are generally connected to a radio frequency coaxial connector through a balun. The pair of spiral conductors is disposed in a plane and the balun is usually contained in a cavity which backs the pair of spiral conductors. An incident electromagnetic wave excites currents in the spiral-shaped conductors. Some of the currents contribute to an inward spiraling electromagnetic wave similar in configuration to the type of wave found on a balanced two-wire transmission line. The balun which is attached to two adjacent inner ends of the spiral conductors converts the balanced wave on the spiral conductors to an unbalanced coaxial TEM wave and transforms the impedance of the spiral conductors to match the characteristic impedance of the coaxial connector and its attached coaxial transmission line. If the radio frequency signal received by the antenna is to be converted to a video signal a detector is employed and is generally attached to the coaxial connector as a separate component.
While the spiral antenna described above is useful in many applications, such structure is difficult to fabricate when operating in a frequency range of between 10 to 100 GHz. More particularly, when such antenna is to operate with millimeter wavelength signals the size of the required balun would be extremely small and difficult to fabricate and the lengths of the transmission line required would be excessively lossy.